This project is a continuation of our earlier research which examined the onsets and development of learning and memory capacities in mice during periods of most rapid central nervous system (CNS) maturation, the neonatal and infantile periods. Based upon recent findings in this laboratory that postnatal treatments of thyroxine administration and undernutrition respectively accelerate and delay the ontogenetic development of memory functions on several learning tasks but have little effect upon the maturation of learning capacities for these tasks, the present project will attempt to assess the effects of thyroxine and undernutrition upon the functional maturation of the cholinergic, serotonergic, and dopaminergic neurotransmitter systems upon the development of learning and memory capacities of tasks requiring response inhibition, such as the passive-avoidance task. BIBLIOGRAPHIC REFERENCES: Nagy, Z.M., & Ritter, M. Ontogeny of behavioral arousal in the mouse: Effect of prior testing upon age of peak activity. Bulletin of the Psychonomic Society, 1976, 7, 285-288. Murphy, J.M., & Nagy, Z.M. Neonatal hyperthyroidism alters the development of behavioral arousal and inhibition in the mouse. Bulletin of the Psychonomic Society, 1976, 8, 121-123.